While both the pectoralis major and pectoralis minor muscles control the movements of the arm, the pectoralis major is specifically responsible for its lateral, vertical and rotational movement. In addition to this primary function, both pectoral muscles also aid in the action of deep breathing.
Each human has two pectoralis major muscles, one on either side of the chest. In addition, two triangular pectoralis minor muscles lie below each pectoralis major muscle and aid them in their functions. In common terms, especially in sports, all of these muscles are collectively referred to simply as the "pecs."
The pectoralis major fans across the chest, beginning at the clavicular head, sternocostal head, and anterior surface of the sternum and inserting at the lateral lip of the intertubercular groove of the the humerus. This allows the muscle, located primarily in the chest, to control the movement of the arm, its major function.
The location of the pectoralis major across the chest, however, also allows it to aid in the movements of deep breathing. In this case, the pecs pull the ribcage out, allowing the lungs room to expand.
In addition, each pectoralis major has six separate sets of muscle fibers that are capable of independent movement through the central nervous system.